doc/meta: Mention --version as a good usecase for installCheckPhase

#### Package tests {#var-meta-tests-packages}

Tests that are part of the source package are often executed in the `installCheckPhase`.

Tests that are part of the source package are often executed in the `installCheckPhase`. This phase is also suitable for performing a `--version` test for packages that support such flag. Here's an example:

Prefer `passthru.tests` for tests that are introduced in nixpkgs because:

```nix

# Say the package is git

stdenv.mkDerivation(finalAttrs: {

  pname = "git";

  version = "...";

  # ...

  doInstallCheck = true;

  installCheckPhase = ''

    runHook preInstallCheck

    echo checking if 'git --version' mentions ${finalAttrs.version}

    $out/bin/git --version | grep ${finalAttrs.version}

    runHook postInstallCheck

  '';

  # ...

})

```

Most programs distributed by Nixpkgs support such a `--version` flag, and it can help give confidence that the package at least got compiled properly. However, tests that are slightly non trivial will better fit into `passthru.tests`, because:

* `passthru.tests` tests the 'real' package, independently from the environment in which it was built

* We can run and debug a `passthru.tests` independently, after the package was built (useful if it takes a long time).